The invention relates to the manufacture of sodium hydroxide.
It relates more particularly to a process for the manufacture of an aqueous solution of sodium hydroxide by electrodialysis.
Electrodialysis is a well-known technique for producing aqueous solutions of sodiumhydroxide. Thus, in U.S. Pat. No. 4,238,305 a process is described in which an electrodialysis cell comprising an alternation of cationic membranes is used, water or a dilute aqueous solution of sodium hydroxide is introduced into the compartments bounded between a cationic membrane and the anionic face of a bipolar membrane and an aqueous solution of sodium carbonate is introduced into the compartments bounded between a cationic membrane and the cationic face of a bipolar membrane. In this known process an alkaline pH is maintained in the compartments which are fed with water or the sodium hydroxide solution and an acidic pH in the compartments which are fed with the sodium carbonate solution. Carbon dioxide is thus generated in the acidic compartments.
In this known process the generation of carbon dioxide in the electrodialysis cell presents disadvantages. In fact, the bipolar membranes employed in these known processes are formed by joining an anionic membrane and a cationic membrane, so that, if carbon dioxide is formed in the pores of the cationic face of the bipolar membrane, the two membranes of which it consists run the risk of parting from each other. On the other hand, the gas thus produced subjects the membranes to mechanical stresses which are liable to damage them and, in addition, significantly increases the electrical resistance of the electrolyte and, consequently, the electricity consumption.
A process for the production of sodium carbonate from trona, by electrodialysis, is described in U.S. Pat. No. 4,592,817. In this known process the formation of carbon dioxide in the acid compartment of the electrodialysis cell is avoided by employing, to feed the latter, a sodium sulphate solution obtained by decomposing sodium carbonate with a solution of sulphuric acid. Since the decomposition of the sodium carbonate is performed by mixing the sodium carbonate with the solution of sulphuric acid in a reactor outside the electrodialysis cell, the carbon dioxide formed in the reactor does not enter the electrodialysis cell. In practice, however, the use of this known process has been found to be difficult, since an untimely release of carbon dioxide in the electrodialysis cell is inevitable in the event of incomplete decomposition of the sodium carbonate.